Скачать или смотреть History of the Atmosphere-Presentation by Ashwin Namburi (Student Chemistry Dept-ibworldwideacademy

Scientists think that the Earth was formed about 4.6 billion years ago.
They think that to begin with, it was a molten ball of rock and minerals.
For its first billion years it was a very hot, turbulent place. The Earth's surface was covered with volcanoes belching fire and gases into the atmosphere.

The Earth's early atmosphere
There are several theories about the Earth's early atmosphere, although
there is little direct evidence to draw on from billions of years ago.
However, scientists have reconstructed what they think the atmosphere
must have been like, based on evidence from gas bubbles trapped in
ancient rocks. They also use data gathered from the atmospheres of other
planets and their moons in the solar system.

One theory suggests that volcanoes released carbon dioxide, CO2, water
vapour, H2O, and nitrogen, N2, and that these gases formed the early
atmosphere. Water vapour in the atmosphere condensed as the Earth
gradually cooled down, and fell as rain. Water collected in hollows in the
crust as the rock solidified and the first oceans were formed. Another
theory speculates that comets could also have brought water to the
Earth. As icy comets rained down on the surface of the Earth, they
melted, adding to its water supplies.

As the Earth began to stabilise, the atmosphere was probably mainly
carbon dioxide. There could also have been some water vapour and
nitrogen gas, and traces of methane, CH2, and ammonia, NH3. There
would have been very little or no oxygen at that time. This resembles
the atmospheres that are known to exist today on the planets Mars and
Venus. Our nearest neighbours have atmospheres made up mainly of
carbon dioxide with little or no oxygen.
After the initial violent years of the history of the Earth, the atmosphere
remained quite stable. That is until life first appeared on Earth.

Oxygen in the atmosphere--
There are many theories as to how life was formed on Earth billions of
years ago. Scientists think that life began about 3.4 billion years ago,
when the first simple organisms, similar to bacteria, appeared. These
could use the breakdown of chemicals as a source of energy.

Then, about 2.7 billion years ago, bacteria and other simple organisms,
such as algae, evolved. Algae could use the energy from the Sun to
make their own food by photosynthesis. This produced oxygen gas as
a waste product. Over the next billion years or so, the levels of oxygen
rose steadily as the algae and bacteria thrived in the seas. More and more
plants evolved - all of them were photosynthesising, removing carbon
dioxide, and making oxygen.
As plants evolved, they successfully colonised most of the surface of the
Earth. So the atmosphere became richer in oxygen. This made it possible
for the first animal forms to evolve. These animals could not make their
own food like the algae and plants could. They relied on the algae and
plants for their food and on oxygen to respire.